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Description  |
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BACKGROUND OF THE INVENTION
The present invention relates to a device for measuring the oxygen
saturation of arterial blood. More particularly, the present invention
relates to an improved non-invasive oximeter and method for mathematically
processing the oxygen saturation calculation independent of pulse
determination.
Oximetry is the determination of the oxygenation level of the blood. One
constituent of human blood is hemoglobin. Hemoglobin which is contained in
red blood cells, picks up oxygen from the lungs and carries the oxygen to
the body cells. Blood traveling from the lungs to the body cells with
oxygen is called arterial blood. Blood traveling to the lungs from the
body cells with diminished oxygen is called venous blood. Oximeters
function by measuring the oxygen saturation, the amount of oxygenated
hemoglobin as a percentage of total hemoglobin, in arterial blood.
The blood oxygen saturation of a patient is one indication of a patient's
pulmonary health. In the operating room, blood oxygen saturation is an
indication of whether an anesthetized patient is receiving sufficient
oxygen. A low oxygen saturation measurement is a warning of dangerous
oxygen deprivation, or hypoxemia, a potential cause of injury or death.
Prior to the development of non-invasive oximeters, the oxygen saturation
of blood was determined "in vitro", commonly in a container called a
cuvette Measurements are first made of the light transmitted through a
cuvette filled with a saline solution. This provides a "bloodless"
reference measurement for use in the oxygen saturation calculation The
cuvette is then filled with blood and a second set of measurements of
transmitted light intensity is taken, to provide "blood-filled"
measurements at two wavelengths The foregoing measurements of light
intensity are converted to absorption values and are then used with
standard equations to solve for blood oxygen saturation
Once non-invasive oximeters were developed, the necessity of taking blood
from the patient was avoided Non-invasive oximeters are now well known and
are used widely to measure oxygen saturation Oximeters function by passing
light of various colors or wavelengths through a sample. On the human
body, typical measuring points are the tip of a finger or an ear lobe. The
sample absorbs the transmitted light to varying degrees relative to the
particular constituents through which the light passes A photosensitive
device, such as a photo multiplier tube or photodiode, is used to detect
the transmitted light Alternatively, the photosensitive device can be
designed to detect the light reflected from the sample. Either system
provides a measure of the light the sample absorbs, i.e., the light the
sample does not transmit or reflect Using measurements of the transmitted
light intensity, the absorption of light by the sample can be calculated.
Calculations can then be made of the percentage of the particular
constituent of interest in the sample.
In general, methods for measuring oxygen saturation utilize the relative
difference between the light absorption (or attenuation) coefficient of
oxygenated hemoglobin and that of reduced hemoglobin. The light absorption
coefficient for oxygenated hemoglobin and reduced hemoglobin is dependent
on the wavelength of the light traveling through them. Both oxygenated
hemoglobin and reduced hemoglobin transmit light having a wavelength in
the infrared region to approximately the same degree However, in the
visible region, the light absorption coefficient for oxygenated hemoglobin
is quite different from the light absorption coefficient of reduced
hemoglobin. The two colors typically chosen to shine through the blood
sample are red light and infrared light. In oximeters, light intensity is
measured at various physiological states The beating of the heart provides
the various states. As the heart beats, arterial blood is forced in the
arteries and capillaries to produce a blood filled state. The blood then
drains leaving a reference which consists of tissue, bone and some amount
of venous blood. The collected transmitted light is subjected to
photoelectric conversion and then mathematical conversion to eventually
calculate the degree of oxygen saturation in the blood.
SUMMARY OF THE INVENTION
The present invention provides an oximeter for non-invasively measuring
oxygen saturation of the arterial blood having a light source of at least
two wavelengths and a detector or detectors for measuring light intensity
after contact with living tissue to produce measurement of at least two
light outputs. Circuit means are provided for processing the light output
signals and a microprocessor for mathematical evaluation of the signals
The processing includes signal separation, noise reduction and
amplification. The processed signals are then used to determine a
mathematical value, based on the variable strength component of the signal
and the steady strength component of the signal for each light output,
from which oxygen saturation can be accurately estimated A novel method
for determining the variable strength component and the steady strength
component of a signal is disclosed.
The recognizable advantage of the disclosed oximeter and method for
calculating oxygen saturation is that finding a pulse is unnecessary. In
other words, if for any reason pulse detection does not work or is not
reliable; a value can still be computed for oxygen saturation. In
addition, this method for calculating saturation uses more of the
information available in the signal and is less sensitive to noise than is
two point calculation o the variable strength signal
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a data stream showing the A.C. component, the D.C. component, and
the D.C offset.
FIG. 2 is the data stream of FIG. 1 following removal of the D C. offset
and following amplification.
FIG. 3 is a noisy data stream with a complicating feature known as baseline
drift.
FIG. 4 is a data stream following removal of the D.C. offset and following
amplification with a complicating feature known as a dicrotic notch.
FIG. 5a is a data stream similar to FIG. 4, but with substantial noise.
FIG. 5b is the filtered data stream of FIG. 5a.
FIG. 6 is a block diagram of the oximeter and wave form filter.
FIG. 7 is a process flow block diagram of the microprocessor unit of the
oximeter and wave form filter.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
The invention can be best understood by first examining typical analog
signals or values of the data points in the data streams following the
analog to digital conversion, outputting from the photo detectors or
photodiodes. In each instance, FIGS. 1-5b, the y-axis represents the
electrical signal, e.g., voltage, and the x-axis represents time. FIG. 1
depicts a relatively noise free data stream without a baseline drift. The
data stream comprises a DC component and an AC component. The DC component
further comprises a DC offset and DC remainder. The AC component relative
to the DC component, is small. To simplify the evaluation of the AC
signal, necessary for the determination of oxygen saturation, the DC
offset is removed. The remaining signal is thereafter amplified. The data
stream of FIG. 1 after removal of the D.C. offset and amplification is
shown in FIG. 2.
The signal of FIGS. 1 and 2 is a relatively clean data stream. However,
many data streams outputting from the photo detectors are substantially
more complicated. FIG. 3 depicts a complex data stream, that is both noisy
and has a substantial baseline drift. Another data stream, shown in FIG.
4, following removal of the DC offset and following amplification has a
complicating feature known as a dicrotic notch. The data stream of FIG. 5a
is similar to that shown in FIG. 4. However, the FIG. 5a data stream has
the additional complicating factor of a high noise level.
The operating principles of the oximeter are described first. One theory of
pulse oximetry holds that blood oxygen saturation can be calculated from
the ratio of two measurements of light attenuation made at each of two
wavelengths. The two measurements form a fraction, based on Lambert-Beer's
law, with the variable strength component of the data stream signal being
the numerator and the steady strength component of the data stream signal
being the denominator, for each of the two wavelengths.
It has been learned that the variable strength component of the signal can
be approximated by the sum of the deviation of the data stream from the
mean of the data stream over a period of time. This is very similar to the
determination of root mean squared measurements of the AC component of the
data stream. One definition for root mean squared is the average of the
absolute deviation from the mean. However, division to find the average of
a ratio, as required by the oxygen saturation calculation, is not
necessary. If 2N+1 represents the number of discrete data points over the
period of time of interest, the mean at a given time t.sub.i is calculated
using the relationship:
##EQU1##
Where P.sub.i+j is the data stream value at time t.sub.i +t.sub.j and
P.sub.i-j is the data stream value at time t.sub.i -t.sub.j.
Once the first mean value is determined, successive means are determined by
the following formula:
##EQU2##
Which is to say that after the first mean is calculated succeeding means
are found by taking the value of the preceding mean and adding the leading
data point in the data stream divided by 2N and then subtracting the
trailing data point for the preceding mean calculation divided by 2N where
the new and old points are separated by 2N points.
For the oxygen saturation calculation, the approximated steady strength
component of the signal, DCI, is the sum of the mean and the DC offset.
The approximated variable strength component of signal, or TIRMS, is:
##EQU3##
ABSV represents the determination of the absolute value of the difference
between P.sub.i and P.sub.i. The TIRMS value is thus a running sum for a
series of absolute values. Each absolute value in the series is the
difference between the mean value and the data point value at a discrete
moment in time. When an oxygen saturation calculation is required, the
TIRMS value can be, but need not be, reset to zero.
Once TIRMS-A and DCI-A, representing the A channel, and TIRMS-B and DCI-B,
representing the B channel, are determined a RATIO is calculated:
##EQU4##
In general, it is conceded that Lambert-Beer's law type absorption and
light scattering by red cells determines the nature of the signals
received by the detector in a typical pulse oximeter transducer. This
being the case, it seems reasonable to use quantities suggested by
Lambert-Beer's law as the basis for the oxygen saturation calculation.
However, it has been learned that whole blood does not obey Lambert-Beer's
law. Therefore, the value of RATIO is not used directly to calculate
oxygen saturation. Rather, both the RATIO and empirically determined data
are used as the basis for making the oxygen saturation calculation. A more
accurate relationship between RATIO and the actual oxygen saturation has
been determined by taking actual measurements of oxygen saturation of
blood and comparing these measurements with the value of RATIO taken
simultaneously. A second order polynomial fit of the data is made. The
polynomial coefficients are dependent on the wavelength of the transmitted
light and several sets of coefficients can be determined as required.
The value of oxygen saturation is thus:
% SaO.sub.2 =a+(b.times.RATIO)+(c.times.(RATIO).sup.2)
where a, b and c are numbers fixed for various wavelengths.
A novel wave form filter based on the following principles can be
incorporated as part of the oximeter Associated with every given data
point in the data stream and equidistant from that data point are multiple
pairs of data points. That is, each data point has multiple pairs of
associated data points. For each pair, the first associated data point
occurs some time prior to the given data point and the second associated
data point occurs an equal amount of time after the given data point. A
difference is found by subtracting the detected values for the associated
data points one from another. The difference for each pair of associated
data points is then summed to form the output of a wave form filter for
the given point. The total time spanned by these associated points is
called the wave form filter length. This can be expressed in the following
fashion:
##EQU5##
Where F.sub.i is the wave form filter output for a given time t.sub.i
having a data stream value P.sub.i, P.sub.i-j is the data stream value at
time t.sub.i -t.sub.j ; P.sub.i+j is the data stream value at time t.sub.i
+t.sub.j ; and the wave form filter length is 2* L+1. Using this approach
to finding F.sub.i requires L subtractions and L-1 additions.
A simpler calculation of F.sub.i+1 is possible if F.sub.i has already been
computed. That is:
F.sub.i+1 =F.sub.i +P.sub.i+L+1 +P.sub.i-L -P.sub.i -P.sub.i+1
This calculation requires only two additions and two subtractions
regardless of the length of the wave form filter. Present microprocessors
are able to make this calculation in real time if the discrete points in
the data stream occur at, for example, 15 millisecond intervals. For this
calculation, memory of 2L+1 values is required.
The accuracy of the output of the wave form filter for pulse detection is
best when the wave form filter length and the pulse length of the signal
are the same. When there is a large mismatch in these two quantities, the
accuracy of the filter is diminished. Two methods have been found to
overcome this problem. The first is to use two or more filters and examine
each of them separately to determine which most closely matches the pulse
length. The other is to combine two filters such that their combined
output will work on any signal of interest. The second method requires
four additions and four subtractions for each point.
FIG. 5b depicts the data stream of FIG. 5a after filtering. Clearly, the
filtering of the data stream eases the pulse determination.
The amplitude of the output from the wave form filter has been found
substantially proportional to the variable signal. The output is therefore
useful in the calculation of oxygen saturation if both an A channel and a
B channel are filtered.
The functioning of the oximeter and the wave form filter is now described.
In FIG. 6, there is depicted a schematic representation of the present
invention. A photoelectric transducer or photodiode 10 receives the light
transmitted through a measuring point in the human body such as an ear
lobe or finger. Two light components are transmitted through the measuring
point. Light component A is transmitted from LED A and light component B
is transmitted from LED B. Both light component A and light component B
are selected for their relative light attenuation in oxygenated hemoglobin
and reduced hemoglobin. In reduced hemoglobin, the attenuation
coefficients of the two light components are substantially different.
Typically red light and infrared light will comprise these light
components.
The data streams detected by photodiode 10 are amplified by pre-amplifier
12 and passed through the synchronized demodulator 14 to separate the data
streams for each of the two light components. For each of the two data
streams, the data stream is further separated (16 and 18) into a DC offset
and the DC remainder plus the AC component. The values of the DC offset
are sampled and held in the microprocessor 40 for further processing.
Alternatively, the DC offset can be preset at a fixed value. Once the DC
offset is removed, data streams A and B are passed through operational
amplifiers 20 and 22. The signal streams are amplified by fixed gains
relative to the signal strengths of channel A and channel B. If the A
channel processes the red signal, the fixed gain may be approximately a
multiple of 200-250 of the preexisting data stream while, the B channel,
if processing an infrared signal, the fixed gain may be approximately a
multiple of 40-60 of the preexisting data stream.
The data streams of both channels A and B are passed through filters 24 and
26 to reduce gross extraneous noise. The signal streams are then passed
through variable attenuators 30 and 32, the control of which is performed
by an evaluation of the signal strength made by the microprocessor 40. The
signal streams are then inputted to multiplexor of 34 where they are
sampled and held until the analog to digital convertor 36 has converted
each incoming analog signal into an outgoing digital signal.
The data streams are thereafter processed as shown in FIG. 7. As each data
point in the data stream is inputted into the microprocessor 40, they are
stored in buffers 52 and 54. As inputs are received the values are
sequentially stored in the buffers replacing previous values which are
shifted through and eventually out of the buffers. When the process is
first started or after data is lost, the microprocessor 40 holds until the
buffer is full before commencing a calculation.
Once the buffers fill, the wave form filter or filter outputs, if more than
one wave form filter is used, are calculated (90, 91). The initial wave
form filter output is determined by subtracting each successive trailing
data point in the data stream from each successive leading data point in
the data stream and then summing values. Each successive wave form filter
output is determined by summing (i) the most recent wave form filter
output, (ii) the trailing data point in the data stream, and (iii) the
leading data point in the data stream for the most recent wave form filter
output calculation and subtracting the sum of (i) the data point in the
data stream halfway through the buffer and (ii) the data point in the data
stream one data point beyond the halfway mark in the buffer. The foregoing
mathematical calculation is performed for each wave form filter used.
Once the wave form filter outputs are determined, a detector 92 is used to
determine an extreme value, such as a maximum or minimum. The rate of the
extreme values are compared (93), by using quality criteria, with an
expected range of values. If the extreme value is within the range of
expected values, the pulse rate is determined (94). The pulse rate is also
compared (95), by using quality criteria, with an expected range of pulse
rates. If the pulse rate is within the range of expected values, the pulse
rate is outputted to the pulse display driver (96).
Simultaneously, the mean values are calculated (56, 58). The initial mean
values for both channels A and B are determined by summing the values of
the data points in the buffer and dividing by the number of data points in
the buffer. After the initial mean values are calculated, additional mean
values are determined by adding the most recent mean value to the value of
the trailing data point in the data stream, divided by the number of data
points in the buffer, and then subtracting the value of the trailing data
point for the most recent mean value calculation divided by the number of
data points in the buffer. Once MEAN-A and MEAN-B are known, TIRMS-A and
TIRMS-B are calculated (64, 66). The mean value is subtracted from the
value of the data point halfway through the buffer to obtain a data stream
comprising intermediate values. The absolute value of the intermediate
values are then summed to obtain TIRMS-A and TIRMS-B.
The TIRMS-A and TIRMS-B values comprise the numerators of the ratios used
in the RATIO calculation. The mean value is also used to determine the
denominators in the ratio calculation. The denominators, referred to as
DCI-A and DCI-B are calculated (74, 78) by summing, separately for each
channel, the mean value and the D.C. offset.
The RATIO calculation is thereafter performed (80). By using the RATIO,
oxygen saturation can be calculated (82). The oxygen saturation
calculation is compared (84), by using quality criteria, with an expected
range of values. If the oxygen saturation is within the range of expected
values, the oxygen saturation is outputted to the oxygen saturation
display driver (86).
A triggering mechanism can be incorporated in the oximeter to initiate each
successive oxygen saturation calculation or an antecedent calculation
required for each successive oxygen saturation calculation. The triggering
mechanism can be a timer or equivalent means. Alternatively, if the wave
form filter is incorporated as part of the oximeter, then the detection of
a pulse can be used as the triggering mechanism.
The TIRMS and mean values are also useful in control of the instrument.
From time to time the microprocessor will adjust the signal strength by
increasing or decreasing the light level being emitted by the LEDs.
Adjustments in the signal strength are necessary because the analog to
digital converter has a limited range, required by the need for precision
and sensitivity in the oxygen saturation calculation. The signal strength
will be adjusted when mean value exceeds an upper or a lower limit. A
signal to the source driver 46 is outputted by the microprocessor 40 by
either channel A or B (60, 62).
The size of the AC component of the signal relative to the size of the
analog to digital conversion range is used to control the variable
attenuators (30, 32). If the AC component of the signal is relatively
small, the signal will be sensitive to digitizing noise resulting in a
loss of accuracy. If the AC component of the signal is relatively large,
baseline drift or other signal variations will cause the signal to move
outside of the range of the analog to digital convertor prompting a change
in the drive current to the LEDs. Changing the drive current to the LEDs
is less desirable than simply attenuating the signal. The attenuators are
controlled by the microprocessor 40. This control function uses the TIRMS
value to determine when attenuation of the signal is required. If so, a
signal is outputted from the microprocessor by either channel A or B (68,
70).
While the above embodiments have been disclosed as the best mode presently
contemplated by the inventor, it should be realized that these examples
should not be interpreted as limiting, because an artisan skilled in this
field, once given the present teachings, can vary from these specific
embodiments. Accordingly, the scope of the present invention should be
determined solely from the following claims.
* * * * *
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